1. Technical Field of the Invention
This invention relates to the formation of structures of incandescent lamp bulbs whose efficiencies have been enhanced.
2. Technical Background
The present inventors et al proposed an incandescent lamp bulb of tubular, transparent shape comprising a visible ray transmitting and infrared ray reflecting film formed on at least one surface of the inside and outside of the bulb, said film being composed of a lamination of alternate high and low refractive index layers consisting respectively of such as titanium dioxide TiO.sub.2 and silica SiO.sub.2, and a tungsten filament centrally and longitudinally disposed in said bulb.
Only visible radiation of the light emitted from the filament of the incandescent lamp bulb passes through the infrared ray reflecting film for emission to the external, while the infrared radiation is reflected by the infrared ray reflecting film to be fed back to the filament to cause it to further heat, thereby improving markedly the incandescent lamp efficiency.
Such a conventional infrared ray reflecting film constitutes substantially a 1/4-wavelength (.lambda.) interference filter so designed as to make the maximum reflection wavelength .lambda. coincide with the peak wavelength (in the approximately of 1.mu.) in the infrared radiation energy distribution of the filament.
Consequently, the lamp efficiency was by no means favorable, because whereas the reflectance for near infrared radiation was fairly good, the visible light transmittance was not taken into account.